Support SSLKEYLOGFILE variable for debugging connections over SSL/TLS

Applications that use libnss for handling TLS, notably Chromium and Firefox ,and some others[1], support setting support environment variable SSLKEYLOGFILE, which writes file of format [2] that allows traffic analyzers like Wireshark to intercept application protocol data. It would be nice if Qt supported this feature as well.

OpenSSL added new feature recently [3]  to allow such interception, with older versions Qt could override some OpenSSL symbols like in [4]

[1] From quick search I've found that mitmproxy supports it, maybe there are others

[2] https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/Key_Log_Format

[3] https://github.com/openssl/openssl/pull/1646

[4]https://git.lekensteyn.nl/peter/wireshark-notes/tree/src/sslkeylog.c (does not work with Qt out of the box, should work with linked OpenSSL though)--